Edition 5.0.5 of the GMP manual, in the section on mpn_mul_1, states:
Don't call this function if S2LIMB is a power of 2; use `mpn_lshift'
with a count equal to the logarithm
of S2LIMB instead, for optimal speed.
Of course, it would be more correct to write "base-2 logarithm" instead
of "logarithm".
-- Regards, Will